Need advice in buying a DSLR?

So I bought a Pentax K-r, less than 2 weeks old amazing features and image quality, but already having problems with "mirror flip, issue" so i want to change it but my budget is £450… I saw Canon 500D / 550D and nikon d3100… The canon 500D comes with (EF-S 18-55mm f/3.5-5.6 DC Lens, Canon EF-S 75-300mm f/4-5.6 III DC Lens) but i heard that there's no IS in the lens and there are not very good… The 550D comes with 18-55mm IS Lens, and Nikon comes with 18-55 VR lens. Any advice of any, make lenses is welcome. ( In the feauture i want to buy other lenses for zoom, macro etc) Please I need your help

Added (1). So it's better repairing it than exchange it with another one?

I had my hands on all 3 in the shops at the weekend. The 500D lenses don't have image stabilization. My favorite was the Nikon D3100 there are plenty of lenses to go with this and there's a double lens kit with both lenses having lens stabilization. 18-55 and 55-200 VR.

Also within your budget is the Panasonic Lumix G3 part of the new four thirds type of camera system, there's no mirror problem with these. No mirrors!

Get your Pentax repaired under Warranty.

Two things I see here from what you wrote and the first responder.

One I'd also recommend getting the Pentax repaired under warranty. If there's a real problem they'll fix it.

Second, it's pretty obvious that first responder has no clue about dslr cameras saying Canon 500D lenses don't have IS? What's up with that? I own two Canon dslr bodies and most Canon lenses are available with IS. Tell this person to get a life. I own a 500D by the way, it's an excellent camera.